With the animal standing, use a cloth measuring tape to calculate the distance from the center of the back down to the underside of the belly (just behind the front legs) and up the opposite side to the center of the back.  Keep the tape flat and comfortably snug—but not tight—for accurate measurement.

Suggestions:

• If you are fitting a puppy that will be a small-sized dog when fully grown, select the small harness for measurements less than 12 inches; the medium harness for measurements of 12" or greater.
• If you are fitting a fully grown dog, select the small harness for measurements less than 14"; the medium harness for measurements of 14" or greater.